In a finding that surprised its authors, a new study revealed that vitamin D deficiency does not contribute to rates of fatal stroke in African Americans, even while it doubles the risk of death from stroke in whites. This, despite the fact that blacks are known to be more vulnerable to vitamin D deficiency than whites, and also to suffer more deadly strokes.

Researchers from Johns Hopkins studied 7,981 adults of both races for 14 years; among the 176 study participants who died of stroke, 116 were white and 60 were black. Despite the absolute numbers, however, the researchers found that blacks were 65% more likely than whites to have a fatal stroke. Also, 32% of blacks in the study were found to have vitamin D deficiency, compared with about 7% of whites — yet the level of vitamin D did not appear to contribute to black’s elevated risk of stroke. (More on Time.com:Black Men With High Blood Pressure, See Your Barber?)

“It may be that blacks have adapted over the generations to vitamin D deficiency, so we are not going to see any compounding effects with stroke,” said co-lead investigator Dr. Erin Michos in a statement.

African Americans tend to have lower vitamin D than whites because their skin color blocks sun exposure, which is necessary for vitamin D production in the body, but some studies have indicated that they have adjusted to lower levels. For example, white patients with low vitamin D often have bone density loss, but the same is not true for black patients. (More on Time.com:Study: Stroke Sufferers More Likely to Die on Weekends)

“Since stroke is the No. 3 cause of death in the United States, it’s important for us to consider low vitamin D as a possible risk factor for stroke at least among whites,” Michos said.

Additional research will be needed to determine why African Americans are so much more likely than whites to die of stroke; hypertension and diabetes play some role, but the researchers say these conditions don’t explain all of the excess risk.

In the meantime, researchers are eager to continue to study vitamin D, which is known to have a number of health benefits, including staving off bone loss, diabetes, some cancers and heart disease. Besides sunlight, sources of vitamin D include fatty fish, egg yolks, and fortified cereals. (More on Time.com:The Hispanic Mortality Paradox: Why Do Latinos Outlive Other Americans?)

The new study was presented at the annual Scientific Sessions of the American Heart Association in Chicago.